Because of the increasingly fast processing power of modern-day computers, users have turned to computers to assist them in the examination and analysis of images of real-world data. For example, within the medical community, radiologists and other professionals who once examined x-rays hung on a light screen now use computers to examine images obtained via ultrasound, computed tomography (CT), magnetic resonance (MR), ultrasonography, positron emission tomography (PET), single photon emission computed tomography (SPECT), magnetic source imaging, and other imaging techniques. Countless other imaging techniques will no doubt arise as medical imaging technology evolves.
Each of the above-identified imaging procedures generates volume images, although each relies on a different technology to do so. Thus, CT requires an x-ray source to rapidly rotate around a patient to obtain up to hundreds of electronically stored pictures of the patient. Conversely, for example, MR requires that radio-frequency waves be emitted to cause hydrogen atoms in the body's water to move and release energy, which is then detected and translated into an image. Because each of these techniques penetrates the body of a patient to obtain data, and because the body is three-dimensional, this data represents a three-dimensional image, or volume. In particular, CT and MR both provide three-dimensional "slices" of the body, which can later be electronically reassembled.
Computer graphics images, such as medical images, have typically been modeled through the use of techniques such as surface rendering and other geometric-based techniques. Because of known deficiencies of such techniques, researchers have turned to volume-rendering techniques as a more accurate way to render images based on real-world data. Volume-rendering takes a conceptually intuitive approach to rendering, by assuming that three-dimensional objects are composed of basic volumetric building blocks.
These volumetric building blocks are commonly referred to as voxels. Whereas, by contrast, the well known pixel is a picture element--i.e., a tiny two-dimensional sample of a digital image have a particular location in the plane of a picture defined by two coordinates--a voxel is a sample that exists within a three-dimensional grid, positioned at coordinates x, y, and z. The voxel has a "Voxel," as that value is obtained from real-world scientific or medical instruments. The voxel value may be measured in any of a number of different units, such as hounsefield units, which are well known to those of ordinary skill within the art.
Using volume-rendering, any three-dimensional volume can be simply divided into a set of three-dimensional samples, or voxels. Thus, a volume containing an object of interest is dividable into small cubes, each of which contain some piece of the original object. This continuous volume representation is transformable into discrete elements by assigning to each cube a voxel value that characterizes some quality of the object as contained in that cube.
The object is thus summarized by a set of point samples, such that each voxel is associated with a single digitized point in the data set. As compared to mapping boundaries in the case of geometric-based surface-rendering, reconstructing a volume using volume-rendering requires much less effort and is more intuitively and conceptually clear. The original object is reconstructed by the stacking of voxels together in order, so that they accurately represent the original volume.
Although more simple on a conceptual level, and more accurate in providing an image of the data, volume-rendering is nevertheless still complex. A key requisite of volume rendering is the use of the entire voxel data set to create an image. In one method of voxel rendering, called image ordering or ray casting, the volume is positioned behind the picture plane, and a ray is projected perpendicularly from each pixel in the picture plane through the volume behind the pixel. As each ray penetrates the volume, it accumulates the properties of the voxels it passes through and adds them to the corresponding pixel. The properties accumulate more quickly or more slowly depending on the transparency of the voxels.
In another method, called object-order (or compositing or splatting), the voxel values are also combined to produce image pixels for display on a computer screen. The image plane is positioned behind the volume, and each pixel is assigned an initial background value. A ray is projected perpendicularly from the image plane through the volume to the viewer. As the ray encounters each successive layer of voxels, the voxel values are blended into the background, forming the image according to each voxel's interpreted opacity. The image rendered in this method as well depends on the transparency of the voxels.
Typically in either method, lighting of the voxel data (i.e., providing for lighting values of the voxel data) is accomplished by conceptually placing a light source coincident with the viewer's perspective, such that each voxel is illuminated insofar as it reflects light back towards the light source (i.e., toward's the viewer's perspective). In most situations, this provides for adequate lighting. However, in some instances, voxel data representing features that have surfaces facing away from the light source are desirable to illuminate, but cannot be provided with lighting insofar as they do not reflect light back towards the light source.
For example, a blood vessel may have an outer wall facing the light source, and an inner wall on the other side of the outer wall that does not face the light source. This inner wall may be desirable to light for clinical diagnosis, but cannot be by the method just described. A light source directed towards the outer wall, that is, provides for illumination of the outer wall because the surface of the outer wall points back to the light source; however, the light source does not provide for lighting of the inner wall because the surface of the inner wall points away from the light source. This is disadvantageous in those situations where lighting both walls is desirable for proper clinical diagnosis, however, and represents an existing shortcoming in current volume-rendering technology.